The invention was made in order to renovate the surfaces of commutators or slip-rings of electric motors that have peripheral circularity defects resulting from the friction of the brushes that maintain a sliding electrical contact with them. It has been observed that these contacts no longer have a uniform radius around their entire periphery after the motor has been in operation for some time. To prolong the life of rotating contacts, limit brush wear and eliminate mechanical vibrations, a repair is made by abrasion resurfacing, typically using a stone or grinding wheel, or by machining of the degraded surface.
This type of repair is usually an expensive operation that immobilises the motor for a long period. With traditional methods, the work requires that the machine should be shutdown for a variable time depending on the power and dimensions of the electric motor and as a function of the difficulties encountered. The duration of such a shut down is typically between a few days and several months. The complete resurfacing operation includes uncoupling of the motor and its mechanical load, removal of the motor and transport to a well-equipped workshop in which the repair itself can be done. In general for powerful motors, the commutator is returned to the supplier for remachining. This has several disadvantages; firstly, a stock of spare motors has to be provided that will be used for the repair time of the disassembled motors; and secondly, reassembly requires special care to achieve difficult realignment of the commutator/armature axis or ring/rotor axis.
To avoid these difficulties an “in situ” repair of rotating contacts is done, in other words in the workshop without disassembly of the part supporting the contact surface to be ground. Typically, motors concerned by this type of “in situ” renovation are electrical motors with a power exceeding 3.5 kW, and in practice up to 650 MW. The “in situ” repair is made by removing accessories such as some brushes and their yokes to access the surface to be ground, and a device comprising a frame is used, the frame being installed on the static part of the motor, with attachment and straight turning means for carrying an abrasion or machining tool and an auxiliary means to drive the motor and thus rotate the rotating contact to be repaired, this means possibly being an auxiliary motor. The combination of the rotation movement of the rotating contact and axial displacements of the tool made using straight turning, resurfaces the degraded surface of the rotating contact. Resurfacing may be done by abrasion or by grinding, or by machining as described in WO00/16945.
WO00/16945 describes an in situ resurfacing process by machining for rotating contacts of electrical machines, that uses a very hard cutting tool, typically made of diamond, and inclined in a special manner from the normal to the surface to be ground passing through the said contact and in which swarf is evacuated by suction close to the tip of the tool.
For example, in order to access the surface of the rotating contact to be ground, one of the brush holder rows is removed, fitted with brushes making the sliding electrical contact between the rotor and the stator. The surface to be ground is in a particular spatial configuration due to the adjacent parts of the motor that have not been disassembled. For example, there is the part of the rotor comprising the winding and the commutator connecting wires or bars, at one end of the commutator. At the other end, there are often static mechanical accessories such as a wall of the motor casing, the bearing or brush holder accessories. Therefore the contact surface of the commutator is like the bottom of a globally concave surface with walls at the ends of the said surface to be ground, perpendicular to the axis of rotation, and with a large radial height. In particular, when resurfacing is done by machining, it is desirable that the resurfacing tool should be held such that the distance between its support and its active end is as small as possible so as to avoid vibrations, or at least to minimise their amplitude. In this case, it is necessary to move the tool holder inside the space delimited by this globally concave surface. However, the presence of these walls at one end and/or at both ends of the commutator makes it impossible to use a single tool to machine the entire surface of the commutator over its entire useful length, due to the size of parts used to hold the cutting tool. Moreover, the ends often have to be chamfered.
Therefore, the objective is to find a fast technical solution that can be used for renovation of the degraded surface of the commutator over its entire useful length, the said useful length being greater than the maximum straight turning displacement of the tool holder.